|a person or thing that appears to be beyond control and is potentially a source of unintentional damage|
One who is uncontrolled and therefore a serious and unpredictable danger. For example, We can't trust her to talk to the pressshe's a loose cannon. This metaphoric expression alludes to cannon mounted on the deck of a sailing ship, which if dislodged during combat or a storm could cause serious damage to both vessel and crew by sliding about. Its figurative use dates from the first half of the 1900s.